1. Field of the Invention
The present invention relates to a polyester composition and, more particularly, to a polyester composition containing a vaterite-type calcium carbonate prepared by a special method, excelled in slipperiness as well as wear resistance and less in the count of coarse projections.
2. Description of the Prior Art
Today, industrially manufactured polyester, especially polyethylene telephthalate (hereinafter referred to as "PET"), is excelled in both physical and chemical properties and is widely in use as fiber, film and other molded articles. In the field of films, for example, it is used in the manufacture of magnetic tape such as audio tape and video tape, for capacitor uses, photographic uses, and packaging and OHP-related uses.
In the field of polyester film, the workability in film-making steps and in processing steps in other uses depends largely on its slipperiness and anti-shaving property, this, in turn, determining the quality of products. In case the slipperiness and/or anti-shaving property are insufficient and when, for example, the surface of polyester film is coated with a magnetic material for manufacture of magnetic tape, friction between the coating roll and the film surface is markedly high as well as the wear of the film surface and, in extreme cases, creases and abrasions are caused to be formed in the film surface. And even after slitting the film coated with the magnetic material, marked friction is caused between many guide parts, playback heads et cetera, this causing separation of chalk-like substance resulting from abrasion, generation of strain, shaving of polyester film surface, and it often results in missing of magnetic recording signals, so-called drop-out.
Hitherto, many methods have been proposed for lowering the friction coefficient of polyester such as incorporating fine particles into polyester to impart a fine and proper degree of roughness to the surface of a molded article but none of them were satisfactory in film's transparency and wear resistance due to insufficient compatibility between such fine particles and polyester. To further explain the method, a large number of methods have hitherto been proposed represented by:
(1) a method in which a part or whole of catalyst et cetera used in polyester synthesis are separated in the reaction process (inner particles separation method), and
(2) a method in which fine particles of calcium carbonate, silicium dioxide and the like are added during polymerization or thereafter (outer particles adding method).
With such particles imparting surface roughness to polyester film, the slipperiness-improving effect attainable is generally larger with increasing size thereof. In precision uses such as magnetic tape and video tape, in particular, however, the increased size of such particles itself can cause defects such as drop-out, hence the surface roughness of film is required to be as small or fine as possible, and required is satisfaction of these contradictory requirements simultaneously.
In the inner particles separation method (1), in which the particles in question are of a metallic salt of polyester component or the like, the compatibility with polyester is relatively good, but it is difficult to control the quantity of particles added and the particle size as well as to prevent formation of coarse particles.
Meanwhile, in the method (2), the slipperiness attainable can be improved by adding fine particles properly selected in particle size and with coarse particles removed by classification and the like and properly controlling the quantity thereof. Since, the compatibility between inorganic fine particles and polyester as organic component is insufficient, however, separation takes place in the boundary between particles and polyester, this giving rise to formation of voids. If such voids should exists in polyester, elimination of particles from polyester film is likely to be caused by contact between polyester films or between polyester film and other media and resultant damage of polyester film or the like, this resulting in, for instance, chalking of the film for magnetic tapes and the aforementioned phenomenon of drop-out. Also, large gaps are bound to exist around the individual particles, this interfering with transparency of polyester film. Hence, the lack of compatibility between inorganic particles and polyester is a problem to be solved for improvement of wear resistance and transparency.
For improvement of compatibility between inorganic particles and polyester proposed is coupling reaction between silane type compound or titanate type compound and inorganic compound, but there have been noted various problems such as complicatedness of the treating process and the effect attainable not so good as expected. Also, for improvement of dispersibility of such inorganic compounds in polyester there has been taken a method of preparing glycol slurry of fine particles of the inorganic compounds and adding it in the course of polyester manufacture. Such inorganic compounds, however, have such defects as being hardly good in dispersibility or and in dispersion stability, being subject to precipitation and deposition and tending to form a hard cake which is hardly redispersible, when glycol with inorganic compounds suspended therein is stored over a long period, and further tending to agglomerate in glycol or in the course of polyester manufacture. The existence of agglomerated coarse particles in polymer causes breaking of yarn during spinning, formation of large projections in film or of fish eyes. Especially when it is used as film for magnetic tape, it gives rise to drop-outs or lowering of S/N ratio, hence development of microparticles without formation of agglomerated coarse particles is awaited.
As one of inorganic compounds used in these outer particles adding methods, monodisperse spherical silica obtainable by hydrolysis, condensation or the like of alcoxysilane is developed and its possibility as anti-blocking agent for video-tape required to have a high degree of quality has been studied.
The aforementioned spherical silica is, however, difficult to prepare economically for the price of alcoxysilane as its material being extremely high and also a long time is required for reaction due to slow progress of hydrolysis and, also, the compatibility of polyester is insufficient compared with those of other inorganic particles added externally to polyester, the aforementioned spherical silica is apt to release from polyester film due to damage of polyester film, this possibly causing chalking or drop-out of film for magnetic tape. Further, since the aforementioned silica is extremely high in Mohs hardness, being 6 or more, there is a large defect of the spherical silica released from polyester film injuring the surface of the reproduction head of the video tape recorder. In this respect, the Mohs hardness of calcium carbonate is approximately 3, this being relatively low among inorganic particles in general. Hence, development of calcium carbonate, same as or better than the aforementioned silica in uniformity and dispersibility and improved in compatibility with polyester has been expected.
Of those fine particles used in polyester manufacture, calcium carbonate, whose material, limestone, yields abundantly in Japan, is being widely used as filler in many fields such as manufacture of paper, paint, rubber and plastic. This calcium carbonate is generally divided into two major categories of heavy calcium carbonate and precipitated calcium carbonate (synthetic calcium carbonate).
Heavy calcium carbonate is calcium carbonate prepared in various grades by classification of mechanically ground limestone, which has a feature of being relatively low in manufacturing cost but broad in particle size distribution and has a defect that by the current grinding-classification technique calcium carbonate higher than a given degree of fineness is unattainable.
Meanwhile, precipitated calcium carbonate is a synthetic calcium carbonate prepared by a chemical method such as carbon dioxide gas carbonation process in which unslaked lime obtained by calcinating limestone at a high temperature is caused to react with water to prepare milk of lime and then letting carbon dioxide gas resulting from calcination of limestone through the milk of lime, lime soda process in which milk of lime is caused to react with sodium carbonate and soda process in which calcium chloride is caused to react with sodium carbonate. Compared with the heavy calcium carbonate obtained by mechanically grinding limestone, this kind of calcium carbonate has a uniform form of primary particle and a relatively good dispersibility. The primary particle form and primary particle size or diameter of precipitated calcium carbonate are arbitrarily selectable by selection of the carbonation conditions such as concentration, temperature, pH, reaction time, and kinds or quantities of additives. The precipitated calcium carbonate, however, tends to have its primary particles cohere or agglomerate to form secondary particles (agglomerates of primary particles) and, therefore, has a defect of being unable to prepare without formation of secondary particles by any conventional carbonation method unless proper mechanical aftertreatment such as wet-grinding by the use of a wet grinder.
These kinds of calcium carbonate are preferably used, for they are relatively low in hardness compared with other fine particles used in manufacture of polyester, and the following methods have hitherto been often used.
(1) A method of repeatedly classifying pneumatically heavy calcium carbonate as it is or with its surface treated with fatty acid, resin acid or one of alkaline metal salts thereof for elimination of coarse particles about 5 .mu.m or more in particle size, this followed by dispersion in glycol before use.
(2) A method of dispersing heavy calcium carbonate in glycol and wet-grinding it by the use of a wet grinder such as a sand mill, this followed by wet classification for elimination of coarse particles about 3 .mu.m or more in particle size before use.
(3) A method of dispersing precipitated calcium carbonate having a specific level of dispersibility and having its particle size in a specific range, this followed by wet-grinding under specific conditions before use.
For use in polyester, however, calcium carbonate prepared by the aforementioned method (1), (2) or (3) has the following fatal defect.
In case of the method (1);
(a) Even if a commercially available fine grade of heavy calcium carbonate is selected for pneumatic classification, the particle size distribution of such heavy calcium carbonate is broad allowing existence of coarse particles 4-6 .mu.m in size. And perfect elimination of coarse particles up to about 3 .mu.m in size is difficult even by repeated classification by the use of a high-level pneumatic classifier. Hence, heavy calcium carbonate prepared by this method is difficult to use in the manufacture of extremely thin polyester film used as material of audio tapes et cetera.
(b) When heavy calcium carbonate surface-treated with fatty acid, resin acid or one of alkaline metals thereof for improvement of classification efficiency of pneumatic classification, the dispersion stability in glycol is deteriorated due to the poor compatibility of such surface treating agent with glycol.
(c) The fineness of heavy calcium carbonate for pneumatic classification is limited, hence preparation of calcium carbonate with its particle size in a given range is infeasible.
In case of (2);
(a) Since in this method heavy calcium carbonate as material is ground by the use of a wet grinder, calcium carbonate with its mean particle size in a given range is feasible with a relative ease compared with the method (1). Wet grinding, however, gives cause for formation of a large amount of fine particles unnecessarily small in size, this resulting in a broad particle size distribution and undesirable decrease of the absolute quantity of calcium carbonate contributing to the desired improvement of friction behavior of polyester film. Worse, such ultra-fine particles re-agglomerate in glycol to form coarse secondary particles, which often deteriorate the physical properties of polyester films or fibers.
(b) Even when heavy calcium carbonate as material is wet-ground by the use of a wet grinder, it is possible that coarse particles 4-6 .mu.m in size are contained in the ground calcium carbonate by the phenomenon called "short pass" (a phenomenon in which coarse particles in the material to be ground are discharged from a wet grinder almost intact) and, even if attempt is made for elimination of such coarse particles by a wet centrifugal classifier, the size of particles economically eliminatable by classification is about 1 .mu.m. Hence calcium carbonate prepared by this method is not usable for polyester films intended for specific uses requiring perfect elimination of particles 1 .mu.m or more in size, for example, 8 mm video tape.
In case of (3);
Since in this method precipitated calcium carbonate used as material has dispersibility higher than a specific level in particle size and degree of dispersion and wet-grinding is carried out under specific grinding conditions, the quantities of coarse particles and unnecessarily fine particles are definitely less than in cases (1) and (2) and calcium carbonate relatively uniform in particle size and sharp in distribution of projections in film surface is obtainable. Even by this method, however, it is difficult to prepare calcium carbonate with no coarse particle about 1 .mu.m in size and to date the final finishing step by centrifugal classification, screening or the like has been taken as indispensable. Further, with the wet-grinder indispensable for the process, partial damage of the grinding media such as glass beads is inevitable, this resulting in undesirous incorporation of traces of broken media 10-30 .mu.m in particle size.